CHEMICAL MODIFICATION OF GAMMA-CARBOXYGLUTAMIC ACID, THE VITAMIN-K-DEPENDENT AMINO-ACID WHICH BINDS CA-2+

.gamma.-Carboxyglutamic acid (Gla), the vitamin K-dependent Ca2+ binding amino acid, can be chemically modified in several specific reactions. Practical outlines of the published procedures for the thermal decarboxylation of Gla to Glu in dry proteins and the specific exchange labeling of the .gamma.-proton of Gla with tritium are presented. The applications of these procedures in the analysis of the role of Gla residues in Ca2+ binding and in biological activity are also discussed. In addition, the reversible formation of pyro-.gamma.-carboxyglutamic acid is described for the first time. Pyro-.gamma.-carboxyglutamic acid is formed from Gla in over 99% yield in a first-order reaction with a half-time of 3.5 h at pH 10 and 110.degree. C. The mass spectrum of the dimethyl ester of the reaction adduct is consistent with pyro-.gamma.-carboxyglutamic acid and treatment of the reaction adduct with 2 M KOH for 24 h at 110.degree. C quantitatively regenerates Gla. The applications of this reversible reaction to the quantitative analysis of Gla in proteins are discussed.